Overvoltage, especially from current surges, can cause capacitors to fail and render entire circuits non-functional.
For example, in automotive applications, the normal DC voltage is between 14 and 12 VDC. Surge conditions such as battery dropout can create overvoltage voltage levels to appear across the bus as high as 37 VDC.
For the regulated 12 VDC applications the recommended capacitor of choice would be a 25 WVDC capacitor. This follows manufacturer's recommendations that the voltage rating of the capacitor be at least twice the rated voltage. This headroom between rated and application voltage allows faults created during surface-mount solder reflow exposures to be undetected during application. If the voltage will be at 14 VDC nominal, such as in typical automotive use, then the recommended capacitor voltage would be at 35 WVDC.
As long as the voltage across the capacitor is maintained near that 50% of rated voltage level, the failure rates will be in the very low PPM. Once that voltage is exceeded, then the PPM level will rise geometrically with the applied voltage level.
All too often this overvoltage is overlooked, and because sample test of 20 to 100 units may not disclose high PPM failure rates, there are many opportunities to dismiss this failure. Ideally, every capacitor should be protected against overvoltage.
In conventional circuit designs, Zener diodes have been used in parallel with capacitors, such as in U.S. Pat. No. 4,100,479.
Combining a Zener diode in parallel with the capacitor, especially a polar capacitor, two potential faults to the capacitor are protected. Firstly, the Zener diode will clamp the voltage to a very specific voltage and prevent voltages above the level from being applied to the capacitor. Secondly, because the Zener depends on reverse or breakdown voltages for the Zener action, it will be forward biased for any reverse voltages, thereby eliminating the potential of applying a reverse voltage to the polar capacitor.
The concept of using a diode in the package with a capacitor to prevent reverse voltages has been presented with the diode in series with the capacitor. Using a Zener in parallel with the capacitor presents a problem in that if these devices are electrically connected before they are packaged the Zener prevents certifying any “headroom” for the capacitor over the Zener's breakdown voltage. The device would be effectively cleared up to the Zener voltage, and subsequent fault sites during the solder process could create faults activated before the Zener voltage is achieved. It is imperative that the capacitor and Zener remain independent of each other until the device is packaged into the packaging system or thereafter. The capacitor must be burned-in and tested to voltages well above the Zener voltage to create the required headroom. Any fault created in the dielectric during the solder process would be highest at the rated voltage of the capacitor, but the Zener is selected in such a manner that the maximum voltage applied to the in-circuit capacitor would be restricted to the Zener voltage, well below the rated voltage of the capacitor.